In the art of metal cutting with band saws, the productivity or efficiency of a machine such as the band saw is the result of a number of variable and controllable factors. Included among such variables are the pressure at which the band saw blade bears against the workpiece, the quality of lubrication of the blade, and the speed at which the blade traverses the workpiece during the sawing cycle.
With regard to blade speed, it is well known that optimum efficiency can be achieved only if the blade speed is set to fit the actual conditions of workpiece characteristics and feed rate. In general, the optimum blade speed is proportional to what is commonly called the "machinability" of the metal workpiece and includes such factors as alloying element content, crystalline structure, and hardness. Thus, for example, readily machinable steels such as structural steel or pipe require moderate blade speeds on the order of about 250 feet per minute, whereas harder steels or cast iron require relatively slow blade speeds on the order of about 50 to 150 feet per minute. Similarly, non-ferrous metals such as brass or copper may be sawed at relatively high blade speeds on the order of about 450 feet per minute.
Skilled workers in the art and operators of band saws also are aware that optimum blade speed and feed pressure are frequently achievable only through the hands-on observational and touch skills of the experienced operator. By way of example, the sound of the cutting action and the appearance of the chips being formed are significant to the skilled operator. The formation of powdery chips could indicate a feed which is too light while short burned chips might result from a feed which is excessive. Similarly, a chattering or vibration of the blade could be the result of excessive blade speed. Thus, whatever blade speeds and feeds may be recommended in published charts and manuals for all of the different types of metals, such recommendations serve at best only as starting rates of feeds and speeds which must be finely tuned to actual conditions for optimization. Optimum sawing conditions thus can be achieved only by the hands-on skill of the operator while the sawing machine is in operation.
Basically, there are two ways of adjusting the speed of a motor-driven band saw blade. One is by use of a variable speed motor, but this is objectionable because of relatively high cost and fluctuating torque. The second is by way of a mechanical variable speed drive employing a belt and sheaves, at least one of which is axially adjustable to vary the drive ratio therebetween and the output speed. The mechanical variable speed drive is more desirable for reasons of lower cost, light weight, easy maintenance and relatively constant output torque. The present invention is directed to such a system.
Band saws incorporating blade speed control systems of the type with which this invention is concerned have heretofore been provided. In this regard, the most material prior art publication is believed to be the publication entitled "Marvel Series 8 MARK I Universal Band Saw" Catalog 803/281 which is published by Armstrong-Blum Manufacturing Co., 5800 West Bloomingdale Avenue, Chicago, Ill. 60639.
As illustrated and described in said publication, the band saw includes a table, a vise for holding the workpiece, a movable blade-carrying column and motor means together with an associated belt system for driving the blade and feeding the same against the workpiece. At the front portion of the band saw, where the operator work station is situated, there is provided a control panel and also the means for adjusting the blade feed pressure. The means for adjusting the blade speed, on the other hand, is mounted on the back of the machine and comprises a direct connection to a variable pitch drive pulley that drives the band wheels.
Continued use and experience with the said prior art band saw has revealed certain disadvantages that pertain to the blade speed control system. For purposes of accuracy, all blade speeds, including the suggested starting speeds recommended in tables and manuals, should be set with the blade drive motor running. With the prior art saw this means that the operator must position the workpiece in the vise, position the band saw blade close to, but not touching, the workpiece, set the suggested feed and turn on the motor while standing at or near the front control panel. The operator is then required to leave the front control panel, go to the back of the machine to set the blade speed indicator at the suggest starting rate, and thereafter return to the front control panel to begin the actual sawing cycle. If further fine adjustment of the blade speed is required, as is normally the case, the operator again has to walk to the back of the band saw to make that adjustment. Not only is the described movement of the operator wasteful of time and energy, but it also takes him away from the usual work station in front of the band saw from where he could best observe and "feel" the efficiency of the cutting operation.
In addition to the foregoing, the prior art band saw had to be placed sufficiently spaced from a room wall to permit access to the back portion of the band saw where the blade speed control is located, an obvious waste of floor space.